Passage of the measure means that prosecutors will no longer have to prove law-enforcement officers acted with “evil intent” — or so-called “malice” — when considering whether to file criminal charges such as manslaughter. Washington is the only state with such restrictive language.

The measure passed with 60 percent of the vote statewide. In King County, support exceeded 70 percent.

In a statement, the I-940 campaign said the win means “Washington becomes the first state in the nation to pass a police training and accountability measure in response to a national conversation about use of force and relationships between law enforcement and the communities they serve.”

I-940 also requires de-escalation and mental-health training for police; requires officers to administer first aid to a victim of deadly force; and requires independent investigations into the use of deadly force.

At one point, state legislators passed a compromise bill earlier this year that addressed concerns raised by some law-enforcement organizations about certain wording in the initiative.

I-940 proponents accepted the bill, agreeing to keep the initiative off the ballot. But the state Supreme Court agreed with a challenger that the initiative couldn’t be modified by the Legislature and must be presented to the voters in its original form.

With its passage, lawmakers can amend it when they return for the 2019 session, although for two years that would require a two-thirds majority.

Both sides have vowed to maintain their support for the modifications.

I-940 requires proof that a reasonable officer would have used deadly force in the same circumstance and sincerely believed the use of deadly force was warranted.

The compromise bill simplified the standard, asking prosecutors only to examine whether a reasonable officer would have deemed deadly force necessary to prevent death or serious physical harm to police or others if placed in the same situation.

Some law-enforcement groups, including the Seattle Police Officers Guild, the King County Police Officers Guild and the Washington State Patrol Troopers Association, opposed the initiative on its face.